Sigma Healthcare Ltd (ASX: SIG) shares surged 9% on Thursday and are the most actively traded stock on their first day of trading post-merger with Chemist Warehouse.

The Sigma share price rose by 9.4% to an intraday high of $3.02 per share today.

The stock has since retraced a little to be trading at $2.94, up 6.52%.

The scheme of arrangement between the two companies was officially implemented yesterday, and about 4.3 billion unescrowed new Sigma shares began trading today.

First day of trading for repackaged Sigma Healthcare shares

At the time of writing, almost 128.5 million Sigma shares have been traded on Thursday, making it the most traded stock of the day, according to CommSec data.

Some of this trading is likely Chemist Warehouse franchisees offloading their holdings.

This blockbuster merger sure has excited the market, and today's share price leap indicates continued optimism for the future.

The Sigma share price has soared since the proposal was announced on 11 December 2023.

The excitement surrounding the deal prompted many investors to buy Sigma shares.

The new demand resulted in Sigma becoming the No. 1 ASX 200 healthcare share for price growth last year.

Sigma shareholders were thrilled with the proposed deal and voted overwhelmingly in favour of it last month.

Today, Sigma is the ASX 200's second-largest healthcare stock with a market capitalisation of $31.84 billion.

Shares held under escrow

Sigma now officially holds all of the ordinary shares in CW Group Holdings Limited, and Chemist Warehouse is now a wholly owned subsidiary of Sigma Healthcare.

A total of 9.9 billion shares were issued as part of the consideration for Sigma's purchase.

However, approximately 5.6 billion remain under escrow.

Chemist Warehouse shareholders who were entitled to receive consideration got just under 45 cents in cash and just over 6.3 new Sigma shares for each Chemist Warehouse share held.

Sigma estimated the market value of those new shares at $2.83 per share.

The 5.6 billion new Sigma shares under escrow were issued in three separate parcels to Chemist Warehouse co-founders Jack Gance and Sam Gance, and CEO Mario Verrocchi.

Sigma said the trio was "committed to the long-term value creation opportunities available to Sigma" post-merger and had therefore agreed to voluntary escrow arrangements.

Subject to certain exceptions, they cannot sell any of their escrowed shares during the First Escrow Period, defined as between now and the earlier of 31 August or the close of trading on the day that Sigma releases its financial results for the period ending 30 June this year.

They cannot sell 90% of their escrowed shares between the end of the First Escrow Period and the earlier of either 31 August 2026 or the close of trading on the day that Sigma releases its financial results for the period ending 30 June 2026.

Jack Gance and Mario Verrocchi, along with Chemist Warehouse's chief property officer Damien Gance, and chief people officer Danielle Di Pilla, have been appointed to the Sigma board.